The present invention relates to an exposure apparatus used in a photolithographic process for producing, for example, semiconductor devices, liquid crystal displays and thin film magnetic heads, among others.
In a photolithographic process for producing semiconductor devices, liquid crystal displays or other products, there have been used exposure apparatuses in which an illumination light beam is utilized for transferring a pattern formed on a photomask or a reticle (a generic term xe2x80x9creticlexe2x80x9d is used hereinafter to mean either), through a projection optical system, onto a silicon wafer or a glass substrate (either is meant by the term xe2x80x9cwaferxe2x80x9d hereinafter) which is photosensitized or coated with a photoresist layer.
In recent years, continuous reduction in linewidth of semiconductor integrated circuits requires higher and higher registration accuracy (i.e., accuracy in alignment between patterns of two layers formed overlaid one on the other on a integrated circuit chip) for exposure apparatus, which accuracy may be, for example, as high as about 50 nm (nanometers). In addition, the size of wafers is increasing from year to year in order to improve productivity, and in fact, 200 mm diameter wafers, which are widely used at present, are being replaced with 300 mm diameter wafers.
Various types of alignment optical systems are used for overlaying an image of a circuit pattern formed on a mask onto a wafer with required registration accuracy, and the practical types of alignment optical systems may be categorized generally into three kinds which are well known in the art. The first is a Through-The-Reticle (TTR) method in which alignment marks formed on a reticle and alignment marks formed on a wafer are observed (or sensed) at a time through the projection lens. The second is a Through-The-Lens (TTL) method in which only alignment marks formed on a wafer are sensed through the projection lens, without sensing alignment marks formed on a reticle. The third is an Off-Axis method in which only alignment marks formed on a wafer are sensed through a microscope system the objective lens of which is disposed at a position distant from the projection lens by a predetermined distance.
Unfortunately, TTR and TTL methods suffer from disadvantage that the alignment light beam, used to illuminate alignment marks, will suffer from chromatic aberration of the projection lens because the wavelengths of the alignment light beam are far different from those of the illumination light beam used for making exposure, and it is a severe technical challenge to correct chromatic aberration of the projection lens with respect to the alignment light beam. The reason why the alignment light beam used for either of these two methods must have such wavelengths is that the alignment light beam should not affect the photoresist layer coated on a wafer, during the alignment process. Further, TTR method suffers from additional problems. For example, TTR method will provide throughput lower than other methods. Further, TTR method has many limitations imposed thereon: for example, TTR method is required to assign wider areas on a reticle to alignment marks formed thereon than any other methods. Due to the above inconveniences associated with TTR and TTL methods, the off-axis method has been most widely used for alignment purposes in recent years.
Nevertheless, the off-axis method has its own drawbacks. For example, the distance between the projection lens (i.e., the projection optical system) and the objective lens of the microscope system (i.e., the off-axis alignment optical system), called the baseline length, may vary under the influence of heat so that the total registration accuracy may be degraded. In other words, the registration accuracy of an off-axis alignment optical system is highly influenced by thermal stability between the projection optical system and the off-axis alignment system. For this reason, the parts or members interconnecting the projection optical system and the off-axis alignment optical system are typically made of low-thermal-expansion materials.
There may be another factor in degradation of the registration accuracy, which factor is the degradation of the measurement accuracy of the laser interferometers used for measuring the position of a stage. The stage may be a stage for carrying a wafer or one for carrying a reticle. While the following description is specifically directed to laser interferometers associated with a wafer stage, it also applies to those associated with a reticle stage. The degradation of the measurement accuracy of the laser interferometers may occur due to a variation in refractive index of air in and around the optical paths of the measuring laser beams. The variation is caused by the air fluctuations arisen from temperature gradient. It is considered that a temperature gradient may occur where a wafer is heated by an illumination light beam illuminated onto the wafer for exposure, and the heated wafer produces an upward stream of relatively hot air rising from around the wafer. One known method for suppressing any variation in refractive index of the air in and around the optical paths of the measuring beams is to produce and maintain a continuous air stream flowing along the optical paths of the measuring beams. Another known method is disclosed by Japanese published patent application No. Hei-2-199814 (No. 199814/1990), in which a local chamber is provided in addition to the main chamber confining the whole exposure apparatus therein, and separate air-conditioning is effected to each of the local and main chambers. This effectively divides the space confined in the local chamber from the remainder in the main chamber, so that the space in the local chamber may be placed under more precise air-conditioning so as to prevent errors in measurements of the laser interferometers.
While it has been long known that registration accuracy of an exposure apparatus may be affected by heat generated in the exposure apparatus, and various solutions to this problem have been proposed as described above, many drawbacks remain in this regard as illustrated below.
First, an exposure apparatus may consist of many parts and members including connection members interconnecting the projection optical system and the off-axis alignment optical system described above, which parts and members are subject to thermal expansions. Using low-thermal-expansion materials to form appropriate parts and members could be one solution if it should be possible. However, in fact, such materials generally have such low rigidities that they are difficult to use for this purpose. For example, a typical low-thermal-expansion glass-ceramic material (such as xe2x80x9cZerodurxe2x80x9d (trademark)) has a sufficiently low thermal expansion coefficient of 0.1 ppm/xc2x0 C. (or 0.1xc3x9710xe2x88x926/xc2x0 C.) or less, but its rigidity is very low. Materials usable to form parts and members of an exposure apparatus are limited to those having sufficient rigidities, such as invert glasses and special ceramic materials. Such high-rigidity materials typically have thermal expansion coefficients of the order of 1 ppm/xc2x0 C. If the required stability in size of members is such that it only allows for a variation of about 1 nm or less in a length of about 0.1 m, then the required stability amounts to within 10 ppb (or 10xc3x9710xe2x88x929). In order to keep this stability in size with materials having thermal expansion coefficients of about 1 ppm/xc2x0 C., the stability in temperature within 0.01xc2x0 C. is required. When conventional heat insulators are used to meet the required stability in temperature, many layers of heat insulating materials have to be overlaid one on another to form such heat insulators. Since the space in an exposure apparatus is so limited, it has been very difficult to achieve the required stability in temperature with conventional heat insulators which can be accommodated in the limited space.
It is to be noted that the limitation on the space in an exposure apparatus of the above type is of great significance as described below. First, reduction of the floor space occupied by an exposure apparatus (this space is often called xe2x80x9cfootprintxe2x80x9d) results in considerable saving of costs for the manufacturer. In addition to the footprint, the space limitation also relates to several essential features having influence over the performance of the exposure apparatus. One of the features is the distance between the projection optical system and the surface of a wafer placed under it. The numerical apertures of projection optical systems have been increasing over recent years for the purpose of achieving higher resolution. If the above distance is relatively large and the projection optical system has a high numerical aperture, this results in large diameters of the optical elements used in the projection optical system. As a result, it is difficult to manufacture projection optical systems. In addition, manufacturing costs thereof increase. Further, a larger projection optical system requires higher bending and torsional rigidities of the structure of the exposure apparatus in order to achieve a registration accuracy within a selected allowable error range, because there are some limitations onto the ratio of length to sectional area for the parts and members composing the exposure apparatus.
As described above, low-thermal-expansion materials are useful for avoiding any adverse effects of heat, however they are very difficult to use as materials of parts and members of an exposure apparatus due to their low rigidity. On the other hand, if materials having sufficient rigidity are used as heat insulating layers, it would be required to provide the heat insulating layers having sufficient thickness in order to meet the required stability in temperature for ensuring avoidance of any harmful thermal expansion. This results in inconvenience with respect to the space in the exposure apparatus. For example, if the required stability in temperature should be achieved with heat insulators of a typical, conventional type, they would have to be as thick as about 10 cm. It is practically impossible to apply such thick heat insulators to the parts and members composing the exposure apparatus. The heat insulators for use in exposure apparatuses fundamentally differ from the heat insulators for other uses, such as in refrigerators, and thus this has been long a problem.
Next we will describe in more detail the problem that the registration accuracy may be degraded when the heat generated in an exposure apparatus affects the measurements of the laser interferometers used in the exposure apparatus.
In recent years, the chamber for confining the exposure machine main portion of the exposure apparatus is typically designed such that it confines not only the exposure machine main portion but also various accessories of the exposure apparatus including power supplies for circuit boards of the controller and electric motors, in order to reduce the required floor space occupied by the exposure apparatus. As a result, the amount of heat generated per unit volume of an exposure apparatus is increasing. Under the influence of such heat sources, it is very difficult to keep the temperature distribution less than 0.1xc2x0 C. This results in a temperature gradient in the exposure apparatus, which in turn produces fluctuations in the air in and around the optical paths of the measuring laser beams from the laser interferometers. Therefor the highest possible stability in measurement through the laser interferometers is limited to 5 nm so far.
The stability in temperature may be improved by confining the wafer stage and the associated laser interferometers in a local chamber and effecting temperature-control to the local chamber by circulating coolant through channels formed in the walls of the local chamber, as described in the above-referenced Japanese published patent application No. Hei-2-199814. This requires, however, a control unit for controlling the temperature of coolant, a circulation pump, piping and other components, resulting in a disadvantageously bulky and complex exposure apparatus occupying an increased floor space (or having an increased footprint).
In view of the foregoing, it is an object of the present invention to provide an exposure apparatus which may prevent the laser interferometers used therein from producing any erroneous measurements, as well as preventing any adverse effects of the heat generated therein, such as degradation of the total registration accuracy, without using low-rigidity materials.
It is another object of the present invention to provide an exposure apparatus which may achieve a reduced footprint (or space-saving) while avoiding any adverse effects of the heat generated therein.
In order to achieve the above objects, the present invention provides the following arrangements for an exposure apparatus.
In accordance with a first aspect of the present invention, there is provided an exposure apparatus which transfers a pattern formed on a mask onto a substrate, comprising: a vacuum heat insulation panel including a filler providing heat insulation in a vacuum condition and a gas-impermeable envelop enclosing said filler in a vacuum condition; and said vacuum heat insulation panel being applied to at least a part of said exposure apparatus.
In accordance with a second aspect of the present invention, there is provided an exposure apparatus including an exposure machine main portion which transfers a pattern formed on a mask onto a substrate and a chamber enclosing said exposure machine main portion and having a temperature therein maintained at a predetermined temperature, said apparatus comprising: a vacuum heat insulation panel including a filler providing heat insulation in a vacuum condition and a gas-impermeable envelope enclosing said filler in a vacuum condition; and said vacuum heat insulation panel being applied to at least a part of a wall of said chamber.
In accordance with a third aspect of the present invention, there is provided an exposure apparatus according to the second aspect as described above, wherein: said vacuum heat insulation panel may be applied to an inner surface of said wall of said chamber.
In accordance with a fourth aspect of the present invention, there is provided an exposure apparatus including an exposure machine main portion which transfers a pattern formed on a mask onto a substrate and a first chamber enclosing said exposure machine main portion and having a temperature therein maintained at a predetermined temperature, said apparatus comprising: a vacuum heat insulation panel including a filler providing heat insulation in a vacuum condition and a gas-impermeable envelop enclosing said filler in a vacuum condition; a second chamber under air-conditioning conducted separately from said first chamber; and said vacuum heat insulation panel being applied to at least a part of a wall of said second chamber.
In accordance with a fifth aspect of the present invention, there is provided an exposure apparatus including an exposure machine main portion which transfers a pattern formed on a mask onto a substrate and a first chamber enclosing said exposure machine main portion and having temperature therein maintained at a predetermined temperature, said apparatus comprising: a vacuum heat insulation panel including a filler providing heat insulation in a vacuum condition and a gas-impermeable envelop enclosing said filler in a vacuum condition; a stage which carries said mask or said substrate; measuring means which emits a measuring beam toward said stage and sensing a reflection of said measuring beam from said stage so as to measure the position of said stage; a second chamber, disposed in said first chamber, which surrounds a space including a space through which said measuring beam passes; and said vacuum heat insulation panel being applied to at least a part of a wall of said second chamber.
In accordance with a sixth aspect of the present invention, there is provided an exposure apparatus including an exposure machine main portion which transfers a pattern formed on a mask onto a substrate and a chamber containing said exposure machine main portion and having temperature therein maintained at a predetermined temperature, said apparatus comprising: a vacuum heat insulation panel including a filler providing heat insulation in a vacuum condition and a gas-impermeable envelope enclosing said filler in a vacuum condition; and said vacuum heat insulation panel being applied to such a region in said chamber whose temperature is higher than said predetermined temperature.
In accordance with a seventh aspect of the present invention, there is provided an exposure apparatus according to the sixth aspect as described above, preferably further comprising: a projection optical system which projects an illumination light beam, illuminating said mask, onto said substrate; said projection optical system having a lens barrel; and said region whose temperature is higher than said predetermined temperature including at least a part of said lens barrel.
In accordance with an eighth aspect of the present invention, there is provided an exposure apparatus according to the sixth aspect as described above, preferably further comprising: a light source emitting said illumination light beam to said mask; a lamphouse confining said light source therein; and said region whose temperature is higher than said predetermined temperature including at least a part of said lamphouse.
In accordance with a ninth aspect of the present invention, there is provided an exposure apparatus according to the sixth aspect as described above, preferably further comprising: a circuit board box containing a circuit board; and said region whose temperature is higher than said predetermined temperature including at least a part of said circuit board box.
In accordance with a tenth aspect of the present invention, there is provided an exposure apparatus which transfers a pattern onto a substrate, comprising: a vacuum heat insulation panel sealed in a vacuum condition; and said vacuum heat insulation panel being applied to at least a part of said exposure apparatus.
In accordance with an eleventh aspect of the present invention, there is provided an exposure apparatus which transfers a pattern formed on a mask onto a substrate, comprising: an acoustic insulator sealed in a vacuum condition; and said acoustic insulator being applied to at least a part of said exposure apparatus.